[fank]

Summary

• Add a local, read-only RCON health check to regular and rootless images.
• Require a valid response within five seconds; missing credentials, authentication failure and unresponsive servers report unhealthy.
• Document Docker health status, tuning and disabling the check. No public RCON or HTTP endpoint is needed.

Closes #532.
Closes #476.

Validation

• All 21 existing regression methods, ShellCheck and whitespace checks pass.
• The health-check implementation is unchanged from the previously tested regular/rootless images: healthy startup, missing/incorrect credentials, and recovery were verified without automatic restarts.
• Removed the added Python test file following maintainer feedback. The PR now changes four files with 25 added lines.